The present invention relates to the recovery of precious metals such as gold and silver from aqueous solutions thereof. Generally, the precious metals, e.g., gold, are present in the form of gold cyanide complexes such as potassium or sodium gold cyanide. Such solutions containing precious metals are obtained or are the by-products of certain industrial processes and it is obviously economically necessary to recover the precious metals values therefrom. For example, in gold plating, sodium or potassium gold cyanide solutions are employed and it is necessary to recover residual gold from the spent plating solution. In mining or in the recovery of gold or other precious metals from scrap or waste material, the precious metal is usually leached from the ore or scrap by a cyanide solution which forms a precious metal cyanide complex. The problem dealt with by the present invention is the recovery of precious metals by precipitating them as elemental metal from solutions containing their ions.
The prior art has devoted attention to this task. For example, U.S. Pat. No. 3,271,135 shows a process for recovering gold from alkali metal gold cyanide solution in which (1) a reducing agent (alkali metal hydrosulfite of hydrazine hydrate), and (2) a compound selected from a group including water-soluble aldehydes and other carbohydrates (e.g., dextrose and formaldehyde) are introduced into the solution. The addition is either simultaneous, or by means of a precursor compound such as sodium formaldehyde sulfoxylate which liberates both components (2and (2). The pantentee notes that component (1) may already be present in the solution when component (2) is added. U.S. Pat. No. 3,311,468 discloses a somewhat similar process specifically for recovery of silver from cyanide solutions thereof.
Another prior art patent, U.S. Pat. No. 3,271,136 seeks to overcome the stated shortcoming of the U.S. Pat. No. 3,271,135 process, which is a tendency to redissolve the precipitated elemental gold in the cyanide solution by adding a water-soluble alkali metal alkanoate and omitting component (2).
U.S. Pat. No. 3,215,524 uses a water-soluble aldehyde both to destroy the cyanide complex and reduce the precious metal ions to elemental metal particles.
The prior art has also separately addressed itself to problems of destruction or conversion of cyanide in aqueous waste for environmental protection. For example, the DuPont Chemical Company has publicized a process under the trademark KASTONE which involves treating cyanide bearing waste water with a formulation containing formaldehyde and a peroxygen compound.
Such prior art processes, when they call for more than one reagent, generally add the reagents simultaneously or do not ascribe importance to the order of addition.
It is an object of the present invention to provide a novel, efficient and economical method for recovering precious metals from aqueous cyanide solutions thereof by first reacting free cyanide ions and then reducing the precious metal ions to elemental metal.
It is another object to provide a method of recovering precious metals from such solutions by a novel stepwise addition of a starter, an accelerator and a clarifier in the order stated.
It is a further object to provide such a method which overcomes certain problems associated with prior art methods and which requires only relatively simple equipment for its practice.
Other objects and advantages of the invention will be apparent from the following description.